This invention relates to radioactive well logging and more particularly to an improved method of and system for distinguishing between oil bearing zones and salt water bearing zones in subsurface formations surrounding a borehole and for further use in determining the porosity of the formations.
In neutron-neutron logging, a steady-state source of primary radiation irradiates the formations surrounding the borehole with neutrons. The resulting secondary radiation can be measured by detectors axially spaced from such source within the borehole. The resulting secondary radiation includes generally the effect of both epithermal and thermal neutron parameters.
The epithermal neutron slowing-down length of the formation is determined primarily by the concentration of hydrogen in the formation and is affected only slightly by the formation's neutron absorption properties. Consequently, the type of fluid in the rock's pore space has little or no effect on the number of epithermal neutrons returning to the borehole as secondary radiation.
The thermal neutron diffusion length of the formation, on the other hand, is affected by the presence of oil or salt water and is significantly reduced when the pore spaces of the formation contain salt water rather than oil. The chlorine present in the salt water has a large capture cross section for thermal neutrons and, consequently, reduces the number of thermal neutrons returning to the borehole as secondary radiation. At the same time, the capture of thermal neutrons by the chlorine effects an increase in the number of thermal neutron capture gamma rays returning to the borehole as secondary radiation.
In U.S. Pat. No. 4,005,290 to Allen, there is described a neutron-neutron logging system for distinguishing between oil bearing zones and salt water bearing zones in subsurface formations surrounding a borehole and for further use in determining oil saturation of an identified oil bearing zone. In such system, a steady-state source of primary radiation is located within a borehole for irradiating the formations surrounding the borehole with fast neutrons. A first pair of detectors located at spaced-apart positions from the source within the borehole measures that secondary radiation affected predominantly by the epithermal neutron parameters of the formation. A second pair of detectors located at spaced-apart positions from the source within the borehole measures that secondary radiation affected by both the epithermal and the thermal neutron parameters of the formation. The first pair of detectors measures the intensity of epithermal neutrons returning to the borehole from the formation.
The second pair of detectors may measure the intensity of either thermal neutrons or thermal neutron capture gamma rays returning to the borehole from the formation. A ratio is taken of the measurements from the first pair of detectors as an indication of the formation porosity. A ratio is taken of the measurements from the second pair of detectors as an indication of both porosity and macroscopic absorption cross section of the formation. An increase in the differential between these two ratios at any given depth indicates a change from an oil bearing zone to a salt water bearing zone in the formation at that depth, while a decrease in this differential indicates a change from a salt water bearing zone to an oil bearing zone. The differential between the two determined ratios is corrected by known factors for the macroscopic absorption cross sections of rock, oil, and water in the formation to give an indication of the amount of oil saturation in an identified oil bearing zone.